An Evaluation of "Nitrogen Saturation": Applicability to Watersheds in Eastern North America including comparisons with two LTER watersheds and results from Japan.

Mitchell, M.J.

SUNY-ESF, Syracuse, NY 13210, USA

Abstract

Retention and loss of nitrogen have received considerable attention due to concerns relating to "nitrogen saturation" in forested watersheds. Generation of nitrate can contribute to surface water acidification and also high levels of nitrate can deleterious effect water potability. The concept of nitrogen saturation has been variously defined but may include chronic losses of nitrate throughout the year and enhanced losses of nitrate during major hydrological events associated with storms and snowmelt. Results will be presented on temporal patterns and N mass balances for selected watersheds in the eastern United States including Hubbard Brook Experimental Forest (New Hampshire) and Coweeta Hydrological Laboratory (North Carolina). These two watersheds are part of the United States LTER (Long Term Ecological Research) network. The results from United States sites will be compared to selected sites in Japan.

Comparisons of seasonal patterns of DIN (ammonium and nitrate) and DON (dissolved organic nitrogen) and mass balances of nitrogen among watersheds both within and among regions suggest that climatological regime, atmospheric deposition of nitrogen, landscape features including slope and the presence of wetlands, land use history, vegetation type and groundwater contribution may all affect the nitrogen dynamics of a watershed. The seasonal pattern of nitrate export in many Japanese watersheds is most similar to the pattern found at Coweeta. The importance of hydrological pathways (e.g., contribution of ground water) and climatic patterns (e.g., warm summer temperatures and abundant moisture) in affecting seasonal patterns of nitrate losses in Japanese watersheds will be presented and compared with sites in the United States and Europe.